A known technique for applying a coating to a plurality of workpieces is to convey the workpieces in a line past a station having one or more spray guns which apply the coating to the workpiece as it passes by. For example, it is common to convey appliances, furniture, automobile body parts and the like to be spray coated, e.g., spray painted, through such a spray station. The spray guns may be stationarily mounted to simply spray the workpiece as it passes by, or mounted on a moveable arm, for example, which is programmed for movement about the workpiece as it passes by. Providing such movement for the spray gun permits the use of fewer spray guns, since the spray guns are mobile and can be variably targeted during a spray program, and allows for more accurate aiming of the spray guns.
A spray gun assembly having a movably mounted spray gun or guns must be capable of effecting fairly quick movements of the spray gun or guns to keep pace with a rapid progression of workpieces being conveyed by. Rapid movement of the spray gun or guns vertically relative to the workpiece is particularly necessary to completely coat the workpiece from top to bottom. That is, the spray gun or guns will have to make a number of vertical passes to coat the side of a wide workpiece which is being conveyed by.
A known way to accomplish rapid vertical, or "Z" axis, movement of a spray gun is to mount the gun on a driven chain which carries the gun around a vertically extending loop, such that the gun makes a vertically upward and a vertically downward pass by the workpiece in each circuit around the loop. The spray gun is typically programmed to spray only during selected intervals when it is passing by the workpiece.
One principal problem with this loop conveyor is that it is not readily adjustable to vary the vertical travel of the spray gun with the vertical dimensions of the workpiece. That is, the stroke of the spray gun is not easily changed, since this requires a change in the size of the chain loop. For example, it is desirable to have a shorter vertical stroke for workpieces with a short vertical height and a longer stroke for workpieces with a long vertical height. The stroke also cannot be adjusted to vary with any change in vertical dimension across the individual workpiece. A car body, for instance, has substantial variations in vertical height along its contour.